VATICAN CITY — Pope Francis will declare the sainthood of Pope John Paul II and Pope John XXIII on Sunday in the first dual pope canonization since the Middle Ages.

John Paul II, born in Poland as Karol Wojtyla, led the Roman Catholic Church from 1978 to 2005, while John XXIII, born Angelo Roncalli in Italy, presided from 1958 to 1963. Both are hugely popular figures among the faithful.

In the past 700 years, only two popes have become saints, Pius V in 1712 and Pius X in 1954, making this weekend’s celebration unique in recent church history.

The Catholic Church makes saints to give the faithful role models. The process is cloaked in secrecy and open to criticism, given that it deals with science-defying miracles and notoriously politicized choices. In Sunday’s dual papal canonization, it also involves rule-breaking, fast-track procedures.

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“Saintliness is part of the church’s DNA,” the Vatican’s current chief saint-maker, Cardinal Angelo Amato, wrote in his 2012 tome on canonization. “Through the centuries, saints have been the spiritual doorway through which humanity is directed toward God.”

A quick look at the sainthood process.

THE BASICS

The Vatican’s detailed process for making a saint usually starts in the diocese where the candidate lived or died. A postulator — essentially the cheerleader spearheading the project — gathers testimony and documentation to build the case and presents the report to the Vatican’s Congregation for the Causes of Saints. If the congregation’s experts agree the candidate lived a virtuous life, the case is forwarded to the pope, who signs a decree attesting to the candidate’s “heroic virtues.”

Over time, the postulator may come across information that someone was miraculously healed by praying to the candidate. If, upon further investigation, the cure cannot be medically explained, the case is presented to the congregation as the possible miracle needed for beatification.

Panels of doctors, theologians, bishops and cardinals must certify that the cure was instantaneous, complete and lasting — and was due to the intercession of the sainthood candidate. If convinced, the congregation sends the case to the pope, who signs a decree saying the candidate can be beatified. A second miracle is needed for canonization.

Beatification allows for veneration of the candidate locally, say in a particular diocese or country. Canonization allows for veneration throughout the universal church.

Martyrs, or people who were killed for their faith, get a free pass and can be beatified without a miracle. A miracle is needed, however, for martyrs to be canonized.

JOHN XXIII AND JOHN PAUL II

John Paul’s record sprint to sainthood started during his 2005 funeral Mass, when chants of “Santo Subito” or “Sainthood Now” erupted from the crowd. Bowing to the calls, Pope Benedict XVI waived the typical five-year waiting period before a saintly investigation can begin and allowed the process to start just weeks after his death.

The rest of the process followed the rules: John Paul was beatified in 2011 after the Vatican certified that a French nun with Parkinson’s disease was miraculously healed after she prayed to him. A Costa Rican woman whose inoperable brain aneurysm purportedly disappeared after she prayed to John Paul was the second miracle needed for canonization.

“I was scared. I just wanted to die at home,” Floribeth Mora said Thursday of her state of mind after receiving her aneurysm diagnosis. She said she saw a photo of John Paul in a magazine on the day he was beatified. “And from that moment, I started a new life.”

John XXIII was beatified in 2000 after the Vatican certified that the healing of an Italian nun who had a gastric hemorrhage was miraculous.

Pope Francis, a spiritual son of John, waived the Vatican rule requiring a second miracle so that John could be canonized alongside John Paul.

While popes past have tended to follow the saint-making process precisely except for occasional exceptions, Francis has waived the rules now on several occasions. On Thursday, for example, he presided over a Mass of thanksgiving for a Brazilian saint he declared without the necessary miracle.

SAINTS GALORE

John Paul declared more saints — 482 — than all of his predecessors combined. Some of his big-name saints: Edith Stein, a Jewish-born Carmelite nun who was killed at Auschwitz; and Maximilian Kolbe, a Polish Franciscan friar who sacrificed his life at the death camp so that a man with a family could live.

He also beatified a record number: 1,338. Among them were John XXIII in 2000 and Mother Teresa in 2003.

Benedict continued, at a slower clip — 45 saints under his watch — and only presided over canonizations, not beatifications. He made one exception for Cardinal John Henry Newman. Benedict beatified the 19th-century Anglican convert to Catholicism during a 2010 trip to Britain.

Francis technically overtook John Paul’s record within two months as pope: In May 2013, he canonized more than 800 15th-century martyrs, the so-called “Martyrs of Otranto,” who were beheaded for refusing to convert to Islam.

TOO MANY SAINTS?

The almost assembly-line approval of saints that started during John Paul’s papacy raised questions that have been reignited with his own record-fast canonization. In his book “Making Saints,” Newsweek magazine’s longtime religion editor Kenneth Woodward argued that the important checks and balances in the saint-making process had been eliminated with the abolition of the “devil’s advocate,” whose job was to challenge the postulator and find the holes in his case.

“Everyone involved in a canonization process now has a stake in its positive outcome,” Woodward said. He said that could result in the process being manipulated and an unworthy candidate canonized. “Without the devil’s advocate, who can prevent such an outcome? And without some means of making the process public, who would know?”

Proponents of the current process insist the checks and balances are in place with the “relator,” or judge who reviews the case.

While few question that John Paul was saintly in many ways, his record-fast canonization has ruffled feathers even inside the Vatican, particularly given the stain on his legacy of having reigned while the sexual-abuse scandal festered.

SHOULD POPES BE SAINTS?

Popes push the sainthood cases they like, ignore the ones they don’t and delay those that are politically inopportune. Look at Oscar Romero, the Salvadoran priest gunned down as he celebrated Mass, a martyr for sure — but his case languished under two papacies that were hostile to liberation theology. Or World War II-era Pope Pius XII, whose case was launched in 1965 but delayed because of accusations by Jews that he didn’t speak out enough against the Holocaust.

Given the politicized nature of the process, some have argued that popes really shouldn’t even be made saints since they can only be models for other popes.

“Making a pope a saint is a way of strengthening his legacy, making it more difficult for future popes to change policies that he put in place,” the Rev. Thomas Reese, a Vatican analyst, wrote recently in the National Catholic Reporter.

But Monsignor Slawomir Oder, the postulator, or chief cheerleader, for John Paul’s case, said it would be “absurd” to exempt popes from possible sainthood since their main job is to spread the faith and encourage Catholics to be saintly.

Before he was pope, John Paul was a student, laborer in a stone quarry, actor, poet, priest, bishop and cardinal.

“John Paul is surely a reference point for his successors, but not just that,” Oder said this week. “You can find the growth of his holiness in all the steps of his life.”